Decisions about whether to build new public transportation projects need to consider how these major investments impact public health. However, to date, research on these impacts has been scarce. To inform future policy making, we propose to evaluate the effects of a new light rail line on health outcomes and health care costs among members of an integrated health plan in Portland, Oregon. We will examine longitudinal data on health biomarkers and health care costs among 4,000 health plan members and matched controls before and after the opening of a new light rail line in 2015. By linking individual behavior, health, and health care costs to parcel-level data on retail outlets, street characteristics, and active transportation infrastructure, we propose to conduct the most comprehensive, robust evaluation of a large-scale community infrastructure improvement to date. We will determine, prospectively, the effect of opening a new light rail line on changes in clinical health measures and health care costs between baseline and three years after the line opens. We will recruit Kaiser Permanente Northwest members aged 18-74 years living within 1.5 km of the new rail line stations; these cases (N=4000) will be compared to a control group (N=4000) matched at the census block group level on demographics, walkability, and transit accessibility. Health measures will include body mass index (BMI), blood pressure, blood lipids, and glycosylated hemoglobin from the electronic medical record (EMR). We will also determine the degree to which any observed changes in health status and costs in the intervention group are mediated by changes in transportation-associated and total physical activity (PA), controlling for walkability (local accessibility), crime and safety measures, pedestrian, bike, recreational infrastructure, and socioeconomic status. This will be done by objectively assessing at baseline and two-year follow up total PA (using accelerometry data) and transportation PA (GPS data), in a randomly selected subsample of 500 exposed and 500 control group members. This study could have a substantial impact on public health policy. Considerable political resistance to increasing public transportation still exists; establishing the health effects and costs of such infrastructure will inform cost-benefit models used to prioritize transportation investments. The rich health and environmental data that our study will yield will enhance public understanding of the ways in which transit investment impacts physical activity and health care costs. This critical information is currently lacking from decision- making about major public transportation investments.